The disclosure herein relates to semiconductor memory devices, and more particularly, to nonvolatile memory devices and to methods of programming nonvolatile memory devices.
Semiconductor memory devices are typically classified as either volatile or nonvolatile depending on their ability to retain stored data in the absence of supplied power. In particular, a volatile memory device loses stored data when supplied power is cut off, whereas a nonvolatile memory device retains stored data when supplied power is cut off. Examples of nonvolatile memory types include flash memory, ferroelectric random access memory (RAM) (FRAM), magnetic RAM (MRAM), and phase change RAM (PRAM).
The type of nonvolatile memory generally depends on the configuration of memory cell transistors thereof. For example, the PRAM is a nonvolatile memory device that uses phase change caused by a temperature change, i.e., a resistance change. The PRAM includes a memory element and a select element. With respect the PRAM, manufacturing processes are relatively simple and memory devices of a large capacity are realized at a low cost.
Flash memory devices typically are divided into NOR flash memory devices and NAND flash memory devices, according to the structure of the cell array. NOR flash memory devices store data through a channel hot electron method and erase the stored data through a Folwer-Nordheim (F-N) tunneling method. NAND flash memory devices stores or erases data through the F-N tunneling method.
A NOR flash memory device has a structure in which each memory cell transistor is separately connected to a bit line and a word line. Therefore, the NOR flash memory device has good random access time properties. A NAND flash memory device has a structure in which multiple memory cell transistors are connected in serial. This structure is referred to as a string structure, requiring one bit line contact for each cell string. Therefore, the NAND flash memory device has good properties in terms of integration.